1. Field of the Invention
The present invention relates to a radio communication system, base station, method of correcting radio link quality information employed therefor, and its program, and more particularly, to a method of correcting radio link quality information informed by mobile stations or terminals to a radio base station.
2. Description of the Prior Art
Data traffic in a mobile communication network has been predicted to show a significant increase as expected in a fixed communication network, a countermeasure against which has been discussed especially for downlink traffic (base station→mobile station: terminal) associated with, for example, downloading from a server.
As one example, HSDPA (High Speed Downlink Access) has been standardized in W-CDMA (Wideband-Code Division Multiple Access).
HSDPA implementations include HARQ (Hybrid Automatic Repeat reQuest) for controlling retransmission. Also, for more efficient control over changes in radio link quality, HSDPA uses methods of time/code multiplexing to achieve transmission of downlink data from a radio base station to a large number of terminals using a common channel.
In addition, HSDPA has a scheduling technique based on radio link qualities of a number of terminals, so that a terminal assigned with a transmission opportunity according to scheduling can receive packet data transmitted with a transmission parameter and transmission power that are set based on the radio link quality of this terminal. The transmission parameter mentioned herein includes the number of Codes, data size, modulation scheme, and the like.
HSDPA employing these techniques uses a common channel by time/code multiplexing to transmit packet data to a number of terminals of various radio wave environments, which means that the more target terminals, the higher system throughput is expected by virtue of a statistical multiplexing effect.
In HSDPA, the radio base station determines the transmission parameter and transmission power of an HS-PDSCH (High Speed-Physical Downlink Shared Channel) which is a common channel used to transmit an allocation schedule of transmission opportunity and packet data.
In order to determine these transmission parameter and transmission power, the radio base station needs to recognize the radio link quality of each terminal. This radio link quality is indicated by CQI (Channel Quality Indicator) information transmitted over an HS-DPCCH (High Speed-Dedicated Physical Control Channel) which is an HSDPA uplink physical channel.
Each terminal measures a reception level of a CPICH (Common Pilot Channel) in an HSDPA Serving Cell, and based on the measured reception level each terminal estimates a transmission parameter by which PER (Packet Error Rate) of the HS-PDSCH becomes “0.1” at the time when the radio base station transmits packet data with HS-PDSCH transmission power of (CPICH)+(known offset value), and then informs the radio base station of the estimate as the CQI information. The known offset value is informed to the radio base station from an upper station (RNC).
By referring to the CQI information informed by the terminal, the radio base station can obtain the transmission parameter of the terminal. The CQI information holds fixed values as shown in FIG. 16 (e.g., see Non-Patent Document 1). In this drawing, a parameter of each CQI value (0 to 30) corresponds to 1 dB step.
The terminal estimates the CQI information according to the following procedures: (1) measurement of the CPICH reception level, (2) estimation of ratio between the measured reception level and interference power [SIR (Signal Interference Ratio)], and (3) notification of the largest CQI value to obtain the PER of HS-DSCH smaller than “0.1”.
The estimation of CPICH reception level includes, for example, CPICH_RSCP (CPICH Received Signal Code Power) measurement of which absolute accuracy requirements range from ±6 to ±11 dB (e.g., see Non-Patent Document 2).
Since the terminal uses the reception level having such an error range to inform of the CQI value that leads to PER of HS-PDSCH (Packet Error Rate of HS-PDSCH) of “0.1”, a measurement error of downlink reception level probably directly affects the CQI value.
FIG. 17 shows a curve of SNR (Signal Noise Ratio)-Log10 (PER) with respect to each CQI value (1 to 30). As is observed from FIG. 17, high sensitivity is shown at PER=0.1 [10Log (PER) =−10] in response to SNR fluctuations in the horizontal axis.
Non-Patent Document 1
3GPP (3rd Generation Partnership Project) TS25.214, V5.3.0 (2002-12), Chapter 6A
Non-Patent Document 2
3GPP, TS 25.133, V5.5.0 (2002-12), Chapters 9.1.1 and 9.1.2
In the foregoing conventional method of informing of radio link quality, when a CQI report value of a given terminal #k is informed higher than the actual CQI value, the terminal #k exhibits an extremely large PER and user throughput is substantially reduced.
In contrast, when the CQI report value is informed lower than the actual CQI value, excessive code and power resources are used for the terminal #k, which causes reduction in system throughput.
The scheduling is based on the CQI report value in HSDPA, therefore requiring high accuracy for the CQI report value. The radio base station thus needs to correct the CQI report value informed by each terminal.
In other words, the conventional method of radio link quality informing uses a radio communication channel with limited code or power resources to transmit packet data. Therefore, the inaccurate report of radio link quality may lead to excessive resource allocation thereby reducing system throughput, or lead to discard of packet data due to insufficient resource allocation thereby reducing user throughput.